Hydrogen addition is an effective way for improving the performance of spark-ignited (SI) engines at stoichiometric and especially lean conditions Spark timing also heavily influences the SI engine performance This paper experimentally investigated the effect of spark timing on performance of a hydrogen-enriched gasoline engine at lean conditions The experiment was carried out on a four-cylinder, port-injection gasoline engine which was modified to be an electronically controlled hybrid hydrogen-gasoline engine (HHGE) by adding a hydrogen port-injection system on the intake manifolds while keeping the original gasoline injection system unchanged A hybrid electronic control unit (HECU) was developed to govern the injection timings and durations of hydrogen and gasoline to enforce the timely mixing of hydrogen and gasoline in the intake ports at the expected blending levels and excess air ratios During the test, the engine speed was fixed at 1400 rpm and the manifolds absolute pressure (MAP) was kept at 615 kPa The hydrogen volume fraction in the intake was increased from 0% to 3% through adjusting the hydrogen injection duration For a specified hydrogen addition level, gasoline injection duration was reduced to ensure the engine operating at two excess air ratios of 12 and 14, respectively The spark timing for a specified hydrogen addition level and excess air ratio was varied from 20 to 50 degrees CA BTDC with an interval of 2 degrees CA The test results showed that the indicated mean effective pressure (Imep) first increased and then decreased with the increase of spark advance The optimum spark timing for the max Imep (OST) was retarded for the HHGE at a specified excess air ratio The max indicated thermal efficiency appeared at the OST. Flame development period was shortened whereas flame propagation period was prolonged with the decrease of spark advance The coefficient of variation in indicated mean effective pressure generally gained its minimum value at the OST HC and NOx emissions were continuously decreased with the retarding of spark timing. However, the effect of spark timing on CO emission was found insignificant (C) 2010 Professor T. Nejat Veziroglu Published by Elsevier Ltd All rights reserved